Plastoquinone compartmentation in chloroplasts. II. Theoretical aspects
1992
Lavergne, J. | Bouchaud, J.P. | Joliot, P.
In the first part of the paper, we describe the rationale that underlies our proposal that rapid plastoquinone diffusion is restricted to membrane domains that include a small number of PS II centers, with a broad distribution of the quinone per center stoichiometric ratio. This hypothesis is shown to account for the non-equilibrium relationship between the redox states of QA and plastoquinones during the photoreduction process. It is strongly supported by the study of the oscillations of the flash-induced oxygen yield. The progressive accumulation of QA(-) causes no additional damping, expressing an all-or-none phenomenon: no significant reduction of QA occurs in a domain until all the secondary acceptors are reduced. The time-course reflects the distribution of the quinone per reaction center stoichiometry, total photoreduction being reached earlier in domains where this ratio is low. In the second part, we propose a structural model in which the domains are caused by the high concentration of proteins in the membrane. From cryofracture data, it appears that the surface fraction occupied by proteins is close to the two-dimensional percolation threshold (0.5), above which a network of closed cells is formed, preventing long-range diffusion of small molecules. A theoretical approach is described, based on percolation theory, that agrees satisfactorily with the experimental data.
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